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| Data Change Map | ||
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| Prev | Building blocks of Volume Replicator | Next |
Data Change Map (DCM) is a bitmap representing the data difference between Primary and Secondary volumes.
Volume Replicator uses DCM for the following:
Performing automatic initial synchronization for the data volumes
Enabling Replicator Log overflow protection when the log protection mode is set to DCM or AutoDCM
Resynchronizing the Primary data volumes using the snapshot
Performing fast-failback
Each data volume in the RVG must have a valid DCM log associated with it before the DCM can be used. Volume Replicator calculates the DCM size based on the size of the volume. The default size of the DCM ranges from 1KB to 256KB depending on the size of the volume. However, you can specify the size of the DCM to a maximum of 2 MB.
Note: |
If you need to resize the data volumes, then Veritas recommends that you also recreate the DCM proportionate to the new size of the data volume. |
When DCM becomes active, the administrator initiates a resynchronization operation and causes Volume Replicator to incrementally synchronize the Secondary with the Primary by looking up the bitmap. Each bit in it represents a region whose contents are different between the Primary and the Secondary. Typically, a region consists of multiples of volume blocks, where each block size is 512 bytes.
Note: |
The Secondary is inconsistent during the period the DCM resynchronization is in progress because the write-order fidelity is not preserved. |
After the resynchronization is complete, the Secondary RVG is consistent and replication resumes with write-order fidelity preserved.
